Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix bug in cfe_enumenv(). The CFE API to invoke is CFE_CMD_ENV_ENUM
and not CFE_CMD_ENV_SET.

Submitted by: neel@

Support for CFE boot loaders. Initially the support is for MIPS, but
other processors will work too.

Submitted by: bms@
Obtained from: p4 mips2 branch